This subproject is one of many research subprojects utilizing the resources provided by a Center grant funded by NIH/NCRR. Primary support for the subproject and the subproject's principal investigator may have been provided by other sources, including other NIH sources. The Total Cost listed for the subproject likely represents the estimated amount of Center infrastructure utilized by the subproject, not direct funding provided by the NCRR grant to the subproject or subproject staff. The simian virus 40 (SV40) T-antigen protein binds the origin of DNA replication and initiates DNA unwinding. This system has been used as a model for understanding the early steps in eukaryotic DNA replication. The structure of the origin-binding domain of SV40 T-antigen has been solved by NMR, but there is as yet no model for how this protein binds DNA. We have produced crystals of the complex between the origin of SV40 replication and the origin binding domain. The structure will help understand how the double hexamer of T-antigen sits on the DNA, and whether origin binding contributes to the helicase activity of the hexameric complex.